In current telecommunications switching systems, multiple directory numbers (DNs) can be assigned to one line. When a call comes into a terminating switching system, a database is searched at that terminating switch to determine the relationship between the incoming call's DN and the equipment number (EN) of the physical line connected to a telephone. The call is then assigned to that line. Conceptually, any number of DNs can be associated with one line. The usefulness of having multiple DNs associated with one line, however, is limited by the currently available facilities at the customer's premises to identify the called DN prior to answering the call, or, preferably, even prior to applying ringing signals to the customer's equipment.
One example in the prior art of informing customers as to the called DN is distinctive ringing or coded ringing (sometimes called "teen ringing"). In this feature, when multiple DNs are associated with a single line, up to four distinctive ringing cadences can be associated with the different DNs. The customer can, prior to answering the phone, but after at least one ringing cycle, determine which DN was dialed by the cadence of the ring. Implementation of this feature, however, requires special high-level service circuits at the switching system to produce the different cadences or tones, and the associated operational software, which are costly as compared to the number of lines using this service.
A further example of the problem of DN identification at the customer premises is in the area of party lines. A great deal of effort and expense goes into constructing line units and designing software to operate line units so that they can accommodate party lines. Some party lines use distinctive ringing, as described above, so that each party on the party line has a distinctive ring. Other line units have special circuitry to provide each telephone on the party line with a private ringing, or "selective ringing" as it is called in the art, by reversing tip-ring for one phone on the party line, etc., as is known in the art. Additionally, there are schemes that encode the ringing signal with various DC signals to provide private ringing for each party on a party line. Special telephones are required to decode this type of ringing signal. Encoded ringing may also be achieved by differentiating the frequency of the ringing signal. All of these schemes require special line unit circuitry and software.
Therefore, a problem in the art is that there is no inexpensive system or method to alert a customer of the terminating directory number for a telephone line associated with a plurality of directory numbers.